The invention relates to an antenna diversity system having at least two antennae for the mobile reception of very-high and ultra-high frequencies [hereinafter: VHF and UHF].
An antenna diversity system with at least two antennae is known having an antenna switch and a tuner to which a selected RF signal is supplied by the antenna switch, the tuner having an output which guides the intermediate-frequency signal or the demodulated signal and is connected with a diversity processor in which a signal evaluation is carried out, the results of which determine whether or not the antenna switch is made to switch through a more favorable input signal to its output via a control line leading from the diversity processor to the antenna switch.
Multipath propagation in mobile radio and television reception causes considerable reception interference. In the case of radio reception this interference greatly impairs listening pleasure due to noise and pronounced distortions of the low-frequency signal and in the case of television reception causes visual disturbance in ghost images, fluttering of the picture, color fading, and loss of synchronization and picture. Such reception interference considerably impairs the reception quality and must therefore be prevented.
An antenna diversity system for eliminating reception interference in frequency-modulated ultrashort-wave radio is known from the German Patent Application P 35 17 247. In this case, at least two antenna signals are supplied to a diversity processor containing a frequency deviation threshold and an amplitude threshold with which the frequency disturbance deviation pulse currently caused by interference and the interference-induced amplitude break-ins in the intermediate frequency signal supplied to the diversity processor from the receiver are compared. If the disturbances exceed the thresholds, a switching process is initiated such that another antenna signal or another linear combination derived from the antenna signals is supplied to the tuner with intermediate-frequency component.
An antenna diversity system for preventing picture interference in the mobile reception of television signals in the VHF and UHF band is described in the German Patent Application P 39 26 336.3. This antenna diversity system has a diversity processor with N antenna signal inputs and a television receiver. The video signal and the horizontal and vertical synchronizing signals are supplied to the diversity processor. The diversity processor contains a time gate which is opened by the horizontal synchronizing pulses during the horizontal blanking time so that the video signal is switched through for evaluating the signal quality. An address signal is generated via a control circuit during incipient picture interference in such a way that a new antenna signal or another linear combination derived from the antenna signals is supplied to the television receiver via an antenna combiner.
Each of the diversity systems described in the patent application mentioned above solves the problem of reducing reception interference in a radio or television channel caused by multipath propagation. But to improve the television sound in a vehicle in addition to the television picture, separate antennae and accordingly also separate antenna diversity systems are required for television reception as well as sound reception in the prior art. The use of the solution indicated in P 39 26 336.3 prevents the picture interference in mobile reception, but not the sound interference of the respective television sound. On the one hand, this is because the television picture is amplitude-modulated in a special manner, while the television sound is frequency-modulated and the criteria for determining the picture quality are completely different in principle than the criteria for determining sound interference. On the other hand, the frequency separation of 5.5 MHz between the picture and sound carriers causes the reception ratios at the location of a reception antenna for picture and sound carriers to be completely different with such a large frequency separation. As a result, e.g., the picture interference can be at a maximum and the sound carrier interference at a minimum simultaneously.
This means that at least four antennae must be arranged on the vehicle when using the aforementioned solutions according to the prior art. But to obtain a distinct improvement in reception a quantity of four antennae per diversity arrangement is recommended in the literature on the subject. This already results in eight antennae which must be mounted on the vehicle according to the prior art to efficiently eliminate picture and sound interference while traveling. Consequently, at least 12 antenna would have to be arranged on the vehicle in order for the stereo television sound transmitted in two bands separated by a gap in frequency to be improved by antenna diversity. Further, considering the enormous frequency band width of approximately 40 to 860 MHz which must be covered, the difficulty of covering this large frequency band with a single antenna presents an additional problem so that the required number of antennae is further increased by the use of band antennae. But it is not possible to accommodate such a large number of antennae on modern motor vehicles.